Yes you can plug a 3 pin fan into a 4 pin connector, you can even plug a 4 pin connector to a 3 pin connector.
The difference is, that a 3 pin connector controls the speed of the fan through the voltage. So the lower the voltage on the 3 pin connectors the slower the fan. If you have a LED fan that also means the LED will go dimmer.
4 pin designs are different, 3 of the pins are still the same voltage. So they can be used on a 3 pin header anyway.
Just the advantage of a full 4 pin header is the 4th pin actually controls the speed in a different way. (you can look up how if you really want to, I won't get too technical here 😉 )
So that way the fan is controlled more accurately and for us, our LEDS don't dim.
So that way either 4 pin or 3 pin connectors are compatible with each other, as long as there is nothing blocking the 4 pin connector.
e.g. plugging a 4 pin into a 3 pin connector, but the connector has plastic on either side of it so the extra hole for the pin will be obstructed by the plastic.
-good luck